iOS: Aphex Xciter is just like your iPhone's standard music app, but it processes the signal to add clarity and richness to your music (and other audio content).

The app just makes most of your music sound better, especially when it comes to songs with a lot going on. I tested the app on various types of music, and I found it dramatically improved anything with a high amount of complexity. With simple songs, on the other hand, it didn't make much of a difference. Basically, it'll work better on an orchestral piece than a piano solo. Either way, it makes your music sound better.

The downside is that it won't play copy-protected tracks, but for the most part that shouldn't be an issue as there isn't much copy-protected music anymore. Also, although you can get a free trial, it only lets you listen for a short time per day. If you want to use the app for an unlimited amount of time, you have to pay $3. This may be a bit much for people who are satisfied with the quality of their music as-is. That said, you can give it a try for free and see what you think. Even if you do want the paid version, download the free one first. At the moment, for whatever reason, the upgrades are cheaper as in-app purchases.